Java The Complete Reference, Seventh Edition

Chapter 24: Using AWT Controls, Layout Managers, and Menus 737

Menu Bars and Menus

A top-level window can have a menu bar associated with it. A menu bar displays a list of

top-level menu choices. Each choice is associated with a drop-down menu. This concept is

implemented in the AWT by the following classes:MenuBar,Menu, andMenuItem.In

general, a menu bar contains one or moreMenuobjects. EachMenuobject contains a list of

MenuItemobjects. EachMenuItemobject represents something that can be selected by the

user. SinceMenuis a subclass ofMenuItem, a hierarchy of nested submenus can be created.

It is also possible to include checkable menu items. These are menu options of type

CheckboxMenuItemand will have a check mark next to them when they are selected.

To create a menu bar, first create an instance ofMenuBar. This class only defines the

default constructor. Next, create instances ofMenuthat will define the selections displayed

on the bar. Following are the constructors forMenu:

Menu( ) throws HeadlessException

Menu(StringoptionName) throws HeadlessException

Menu(StringoptionName, booleanremovable) throws HeadlessException

Here,optionNamespecifies the name of the menu selection. Ifremovableistrue, the

menu can be removed and allowed to float free. Otherwise, it will remain attached to

the menu bar. (Removable menus are implementation-dependent.) The first form creates

an empty menu.

Individual menu items are of typeMenuItem. It defines these constructors:

MenuItem( ) throws HeadlessException

MenuItem(StringitemName) throws HeadlessException

MenuItem(StringitemName, MenuShortcutkeyAccel) throws HeadlessException

Here,itemNameis the name shown in the menu, andkeyAccelis the menu shortcut for this item.

You can disable or enable a menu item by using thesetEnabled( )method. Its form is

shown here:

void setEnabled(booleanenabledFlag)

If the argumentenabledFlagistrue, the menu item is enabled. Iffalse, the menu item is

disabled.

You can determine an item’s status by callingisEnabled( ). This method is shown here:

boolean isEnabled( )

isEnabled( )returnstrueif the menu item on which it is called is enabled. Otherwise, it

returnsfalse.

You can change the name of a menu item by callingsetLabel( ). You can retrieve the

current name by usinggetLabel( ). These methods are as follows:

void setLabel(StringnewName)

String getLabel( )

Here,newNamebecomes the new name of the invoking menu item.getLabel( )returns the

current name.